how to convert a srting into italics

i have a string "my new_number_1234". Want to convert it as "my new_ number_1234_". How can i??????

[Walter Roberson]

Where would the italics have to show up?

Strings themselves do not have slant (italics) or weight (bold) or color. The presentation of strings might have any of those. But we have to know your presentation method to tell you how to convince that presentation method to use those decorations.